Vortex Dynamics in Atomic BECs: Some Recent Developments

Leslie O. Baksmaty, Sungjong Woo, Stephen U. S. Choi

Kali Wilson

Bose-Einstein condensates (BECs), with their superfluid behavior, quantized vortices, and high-level of control over trap geometry and other system parameters provide a compelling environment for studies of quantum fluid dynamics. Recently there has been an influx of theoretical and numerical progress in understanding the superfluid dynamics associated with two-dimensional quantum turbulence, with expectations that complementary experiments will soon be realized. In this dissertation I present progress in the development of an experimental toolkit that will enable such experimental studies of two-dimensional quantum turbulence. My approach to developing this toolkit has been twofold: first, efforts aimed at the development of experimental techniques for generating large disordered vortex distributions within a BEC; and second, efforts directed towards the design, implementation, and characterization of a quantum vortex microscope. Quantum turbulence in a superfluid is generally regarded as a disordered tangle of quantized vortices in three dimensions, or a disordered planar distribution of quantized vortices in two dimensions. However, not all vortex distributions, even large disordered ones, are expected to exhibit robust signatures of quantum turbulence. Identification and development of techniques for controlled forcing or initialization of turbulent vortex distributions is now underway. In this dissertation, I will discuss experimental techniques that were examined during the course of my dissertation research, namely generation of large disordered distributions of vortices, and progress towards injecting clusters of vortices into a BEC. Complimentary to vortex generation is the need to image these vortex distributions. The nondeterministic nature of quantum turbulence and other far-from-equilibrium superfluid dynamics requires the development of new imaging techniques that allow one to obtain information about vortex dynamics from a single BEC. To this end, the first vortex microscope constructed as part of my dissertation research enabled the first in situ images of quantized vortices in a single-component BEC, obtained without prior expansion. I have further developed and characterized a second vortex microscope, which has enabled the acquisition of multiple in situ images of a lattice of vortex cores, as well as the acquisition of single in situ images of vortex cores in a BEC confined in a weak hybrid trap. In this dissertation, I will discuss the state-of-the-art of imaging vortices and other superfluid phenomena in the University of Arizona BEC lab, as indicated by the examined performance of the quantum vortex microscope.

Aaron James Sup

Tōru Morishita, Shinichi Watanabe

We are interested in interplay between nonlinearity and non-adiabaticity in many-body quantum systems: Bose-Einstein condensates (BEC). Adiabatic (geometric) phases and dynamics of adiabatic invariants are conceptual issues in classical mechanics and have been used recently in studying dynamics of BEC and ingenious manipulations with it. We present here two our recent results related to this theme. The first concerns a vortex creation during splitting of a spinor BEC in a magnetic chip-based trap. Our study is motivated by a recent MIT experiment 1 , where a fork-like structure appeared in the interference fringes indicating the presence of a singly quantized (SQ) vortex in one of the interfering condensates. It is well-known that in a spin-1 BEC in a quadrupole trap a doubly quantized (DQ) vortex is topologically produced by a “slow” reversal of bias magnetic field Bz. This fact can be explained using an adiabatic (Berry) phase argument. Since in the experiment a DQ vortex had never been seen, Y. Shin et. al ruled out the topological mechanism and concentrated on the mechanical mechanism for explanation of the vortex creation. We find however that both mechanisms are possible: SQ vortices can be formed in a spin-1 BEC topologically during the magnetic field switching-off process 2 . We therefore provide a possible alternative explanation for the interference patterns observed in the experiment. The second example concerns dynamics of atom-molecule BEC systems being considered recently, in particular in relation to Feschbach resonance passage. We use rigorous separatrix crossing theory 3 to study dynamics of adiabatic invariants in corresponding models. Our results 4 contradict those of 5

Nicola K. Wilkin

Zhai Hui, Qi Zhou

When the repulsive interaction strength between atoms decreases, the size of a rotating Bose–Einstein condensate will consequently shrink. We find that the rotational frequency will increase during the shrinking of condensate, which is a quantum mechanical analogy to ballet dancing. Compared to a non-rotating condensate, the size of a rotating BEC will eventually be saturated at a finite value when the interaction strength is gradually reduced. We also calculate the vortex dynamics induced by the atomic current, and discuss the difference of vortex dynamics in this case and that observed in a recent experiment carried out by the JILA group [Phys. Rev. Lett. 90 (2003) 170405].